System for freeway access ramp traffic control



May 21, 1968 L. F. WALDRON SYSTEM FOR FREEWAY ACCESS RAMP TRAFFICCONTROL 3 Sheets-Sheet 1 Filed Jan.

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INVENTOR. L. F WALDRON HIS ATTORNEY ay 21, 1968 L. F. WALDRON SYSTEM FORFREEWAY ACCESS RAMP TRAFFIC CONTROL 3 Sheets$heet 2 Filed Jan.

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590 A G E 6 29555 m 50 0 o 566 Gamma. m @E HIS ATTORNEY Unite StatesPatent 3,384,869 SYSTEM FOR FREEWAY ACCESS RAMP TRAFFIC (IONTROL LeslieF. Waldron, Rochester, N.Y., assignor to General Signal Corporation,Rochester, N.Y., a corporation of New York Continuation-impart ofapplication Ser. No. 346,177, Feb. Zil, 1964. This application Jan. 17,1%6, Ser. No. 521,184

Claims. (Cl. 34036) ABSTRAQT OF THE DISCLOSURE A system for controllinga signal governing ramp traffic into a controlled access highway havingdetectors in the highway and metering timer means controlled by thedetectors for governing the permissible proceed time for the signal inaccordance with trafiic conditions. A detector in the ramp between thesignal and the highway is used at times to restore the signal to stop.

This application is a continuation-in-part of my application Ser. No.346,177, filed Feb. 20, 1964, now abandoned.

This invention relates to traffic control systems, and it moreparticularly pertains to a system for regulating entry of trafiic onto afreeway from an access ramp in accordance with freeway trafficcharacteristics.

Modern high-speed freeways require free-flow of traffic in order toavoid congestion resulting ultimately in traffic stoppages. Under heavyfreeway traffic conditions, entry of vehicles onto a freeway withoutlimitation may ultimately result in freeway traffic stoppages. Underlight freeway traffic conditions, however, it is desirable to permitvehicles to enter onto the freeway with a minimum amount of limitation.Under freeway traffic conditions ranging between light and heavy, it isdesirable to limit access to the freeway from a ramp in a graduatedmanner; that is, as freeway traffic increases from light to heavy, forexam-ple, an increasing limitation on vehicles entering the freeway froma ramp is desirable. By so doing, traffic conditions on the freeway maybe maintained within predetermined maximum limits, so as to avoid a highdegree of congestion, resulting in stoppages.

The system according to the present invention provides for the controlof a ramp signal selectively according to two modes. One of these modesis a normally clear mode wherein substantially unlimited trafiic ispermitted to enter the freeway from the ramp when there is no congestionof traffic on the freeway, and the second mode is a normally stop modewherein a traffic signal governing the entry into the freeway from theramp is normally at stop, and is permitted to indicate proceed to startvehicles into the freeway at metered intervals at a rate selected inaccordance with the extent of congestion of traffic on the freeway. Inthe normally stop mode, the signal is permitted to indicate proceed onlyfor the interval required for the passage of a vehicle past the signalbecause means is provided for detecting the vehicle having passed thesignal, and such means causes the signal to be put to stop. A signalthus put to stop is maintained in a stop indicating position for aninterval dependent upon the degree of congestion of traflic on thefreeway. In one form of the invention, it is further required that avehicle be in the immediate approach of the ramp traffic signal beforeit is given a proceed indication by the signal.

In accordance with the operation of the ramp traffic signal selectivelyin a normally clear or a normally stop mode, a special warning can beprovided in accordance with the display of a distinctive signal aspect,for exam- Patented May 21, 1968 "ice ple, when the system changes from anormally clear mode to a normally stop mode.

An object of the present invention is to provide means for automaticallyselecting a timing cycle for a ramp traffic signal in accordance withmeasured trafiic characteristics such as characteristics related totraflic congestion on a freeway.

Another object of the present invention is to provide means forcontrolling a ramp traffic signal to admit traftic onto a freeway from aramp at a metered rate selected in accordance with trafiiccharacteristics on the freeway.

Another object of the present invention is to control a ramp trafficsignal selectively in normally clear and normally stop modes, dependentupon the traflic characteristics on the freeway.

Another object of the present invention is to control the ramp tratlicsignal, under conditions where traffic on the freeway is congested, in amanner to require the presence of a vehicle substantially immediately inapproach of the traffic signal before the signal is permitted to displaya proceed indication.

Another object of the present invention is to selectively determineminimum durations between proceed intervals of the ramp trafiic signalin accordance with the congestion of traffic, and to limit the proceedintervals in accordance with the sensing of traffic having passed theramp signal.

Other objects, purposes and characteristic features of the presentinvention will be in part obvious from the accompanying drawings and inpart pointed out as the description of the invention progresses.

In describing the invention in detail, reference is made to theaccompanying drawings, wherein similar reference characters are usedthroughout the several figures to designate corresponding parts andwherein:

FIG. 1 illustrates a system for the control of a ramp signal accordingto one embodiment of the present invention;

FIG. 2 illustrates a second embodiment of the present invention whereinthe ramp signal can be selectively controlled either manually orautomatically to provide the desired mode and metering selection; and

FIG. 3 illustrates another embodiment of the present invention employingadditional trafiic sensing means in the access ramp for use incontrolling the ramp traffic signal during transition from a normallyclear to a normally stop mode.

With reference to FIG. 1, an access ramp is shown leading onto afreeway. A plurality of vehicle detectors are situated so as to sensetraffic characteristics which can be considered as generally relating totraffic congestion on the respective lanes of the freeway upstream fromthe location at which the access ramp empties into the freeway, althoughit is to be understood that these detectors can be located at otherpoints in the freeway in accordance with the requirements of practice.One example of a detector which may be used with this system isdisclosed in the Kendall et al. Patent No. 3,042,303, issued July 3,1962. Although three detectors A, B and C are illustrated, a singledetector may also be utilized, especially if the assumption is made thatfreeway traffic tends to distribute itself substantially equally overall lanes. On the other hand, a single detector may be used if preferredto monitor traliic only in the lane into which the access ramp trafiicempties directly.

Outputs of dectors A, B and C may be registered in any suitable manner,such as in the manner illustrated wherein the relays 11, 12 and 13 arecontrolled by the detectors respectively and have respective frontcontacts 14, 15 and .16 which provide separate inputs to a trafiiccongestion computer '17. If the degree of congestion is to be determinedin accordance with lane occupancy, the

computer 17 can be provided in a manner described in the application ofJ. H. Auer, Jr., ct al., Ser. No. 292,584,

led July 3, 1963. If this type of a computer is used, the voltageamplitude produced by the computer is proportional to the average oflane occupancy data supplied to its inputs. On the other hand, ifdetectors A, B and C are to sense tratfic congestion by measuring thevolume of tratfic, computer 17 may comprise a volume computer such asthat shown in the J. H. Auer, Jr., et al application Ser. No. 342,467,filed Feb. 4, 1964. in such case, output voltage amplitude produced bythe computer 17 is proportional to the average of volume data suppliedto its inputs.

Output voltage produced by traffic congestion computer 17 is applied toa level monitor 18. In FIG. 1, the level monitor .18 utilizes aplurality of output contacts 19, 20, 21 and 22. The heel of contact 19is coupled to a source of energy, while back contacts 19, 20, 21 and 22,as well as front contact 22 are respectively coupled to key operatedcontacts 34, 35, 36, 37 and 38 of a dial unit 23. The dial unit 23,which is preferably driven by a IUD-segment dial geared for sixty-secondoperation, permits preselection of a wide variety of closed contactintervals, dependent upon key selection and placement. Front contact 19is coupled to the heel of contact 20, front contact 20 is coupled to theheel of contact 21, and front contact 21 is coupled to the heel ofcontact 22.

Dial unit output energy is coupled to a relay 24 having a pair ofcontacts 25 and 26. Contact 26 supplies energy to the indicator lamps ofa traflic signal 27 regulating entry of trafiic from the access ramponto the freeway. Back contact 26 provides energy to the red lamp, whilefront contact 26 provides energy to the green lamp. Front contact 25provides a stick circuit for relay 24 through a back contact 28 of arelay 29. The system as illustrated in FIG. 1 is shown as being in anormally clear mode wherein no traffic congestion is detected on thefreeway, the green lamp of signal 27 being energized to provide aproceed indication for the ramp signal in accordance with the closure offront contact 26 of relay 24.

A suitable vehicle detector D is situated just beyond traffic signal 27for the purpose of sensing vehicles which have passed signal 27. Outputenergy from detector D is coupled to a relay 30 having a contact 31. Theheel of contact 31 is coupled to a capacitor 32. Front contact 31receives energy through a current limiting resistor 33, while backcontact 31 is coupled to relay 29.

In operation, detectors A, B and C sense traffic conditions on thefreeway and actuate relays 11, 12 and 13 accordingly. Computer 17 thenreceives energy in accordance with the operations of relays 11, 12 and13, and provides an output proportional to tratfic characteristics suchas trafiic congestion on the freeway. The computer output voltage, whichmay represent an analog of freeway lane occupancy or freeway volume,dependent upon whether a lane occupancy computer or volume computer isused, or some other congestion parameter, is supplied to a level monitor18 which closes its front contacts in stepwise fashion in accordancewith changing amplitude of input voltage supplied thereto. Hence, whenoutput voltage supplied by computer 17 to monitor 18 is at a minimumlevel, back contacts 19, 20, 21 and 22 are all closed. Under theseconditions, back contact 19 supplies energy to dial unit 23. However, asthe amplitude of computer 17 output voltage increases, front contact 19closes, thereby providing output energy from level monitor 18 at backcontact 20. As computer 17 output voltage increases further, frontcontact 20 closes, while front contact 19 remains closed. Hence, outputvoltage is provided at back contact 21. If voltage supplied to levelmonitor 18 increases still further, front contact 21 closes, while frontcontacts 19 and 20 remain closed, so that output voltage is supplied atback contact 22. Ultimately, at maximum levels of output voltageproduced by computer 17, front contacts 19, 20, 21 and 22 of levelmonitor 18 all close, so that output voltage is provided at frontcontact 22 of the level monitor. In the foregoing fashion, level monitor18 classifies amplitude levels provided by traffic congestion computer17, supplying, in effect, analog to digital conversion.

As output voltage amplitude produced by traffic congestion computer 17increases, energy is transferred from one dial unit 23 contact toanother. It can be seen that only One dial unit 23 contact may beenergized at any given time. Moreover, each dial unit 23 contact remainsclosed for a different length interval during each timing cycle. in thisfashion, therefore, relay 24 receives energy from dial unit 23 duringdifferent length intervals, dependent upon the amplitude of outputvoltage produced by tratfic congestion computer 17.

Normally, with no traffic present on either the ramp or the freeway asis illustrated in FIG. 1, the relay 24 is maintained in its picked upposition by its circuit through back contact 28 of relay 29 and its ownfront contact 25, and in accordance therewith, the green lamp G ofsignal 27 is energized to display 2. proceed indication through frontcontact 26. The energization of relay 24 by this circuit maintains therelay 24 steadily energized as long as no traffic is sensed havingpassed the signal 27, irrespective of the pick up circuit for the relay24 being intermittently opened by the operation of the dial unit 23. Onthe other hand, while relay 24 is energized from the dial unit 23, anynumber of vehicles may pass detector D without causing a change inindication from,

green to red. Assuming now that energy is removed from relay 24 by thedial unit 23, as long as no vehicle is sensed by detector D, relay 24remains energized by energy through its front contact 25. However, whena vehicle is sensed by detector D during the time in which dial unit 23provides no energy to relay 24, the relay 24 becomes dropped away inaccordance with the momentary operation of relay 29. Relay 30 ismomentarily energized by the detector D, and this relay actuates itscontact 31 to first charge the capacitor 32, and then upon the droppingaway of relay 30, to momentarily pick up relay 29 to open the stickcircuit for relay 24 at back contact 28. The dropping away of relay 24shifts its contact 26 from a front to a back position, thus causing thedeenergization of the green lamp G of signal 27 and the energization ofthe red lamp R of that signal. Thus the ramp signal 27 governing vehicleaccess to the freeway from the ramp is changed from a proceed indicationto a stop indication.

It will be noted that under conditions in which heavy traffic enters thefreeway from the access ramp, and depending upon the amplitude of outputvoltage provided by computer 17, the traffic signal 27 regulating entryof vehicles onto the freeway is controlled by the dial unit 23 so as tolimit passage of vehicles on the freeway only during those predeterminedintervals established in the dial unit 23 and selected by the levelmonitor 18 in accordance with amplitude of the computer 17 outputvoltage. However, under light access ramp trafiic conditions, thetraflic signal 27, when in the proceed condition, may remain in theproceed condition until detector D senses passage of a vehicle during aportion of the cycle of the dial unit 23 in which the dial unit is notsupplying energy to relay 24. In a sense, therefore, the end of theproceed interval, under light access ramp traffic conditions, isvariable, and may actually extend into the next timing cycle to providea substantially continuous proceed indication. It should also be notedthat detector D may be of the same type as detectors A, B and C, or ofany other well-known type.

With reference to FIG. 2, another embodiment of the invention isdisclosed wherein the system more definitely operates either in anormally proceed mode or in a normally stop mode as selected inaccordance with the detection of congestion of trafiic on the freeway.If there is no traffic congestion on the freeway, the access ramp signal50 steadily displays a proceed indication by energization of the greenlamp G of that signal through front contact 51 of relay SC2 and backcontact 52 of relay SCI.

The sensing of trafiic congestion on the freeway for the systemaccording to FIG. 2, can be by a traffic congestion computer 17 the sameas has been described with reference to FIG. 1. The output of thetraffic computer 17 of FIG. 2. is applied to a level monitor 53, whichis similar to the level monitor 18 of FIG. 1. The level monitor 53 hasoutput contacts 54, 55 and 56 which are progressively actuated to theirpicked up positions in accordance with the increase in trafficcongestion along the freeway. The output of the level monitor 53 isapplied through a suitable multiple position switch 57 which can beoperated manually to select either the automatic operation of the systemin accordance with freeway congestion as detected by the detectors A, Band C, or the different degrees of congestion can be manually designatedby the switch 57 for use in controlling the ramp trafiic signal 50. Acode characteristic of the degree of traffic congestion is thus appliedover code wires 58 and 59 of relays C1 and C2. It is to be understoodthat such a control switch can also be used with other embodiments ofthe present invention.

A suitable indicator 60 is provided for indicating to an operator thedifferent levels of congestion that are detected on the freeway, or toindicate the ditferent levels of congestion that are designated manuallyby the switch 57, if the switch is operated to positions other than theautomatic position in which it is shown. With the switch 57 in theautomatic position, and with no congestion detected on the freeway, thelamp of indicator 60 is energized through back contact 54 of levelmonitor 53 and contact 61 of switch 57 in its automatic position. Thereis a lamp illustrated in the indicator 60 for registering levels 1, 2and 3 of traffic congestion respectively. If congestion is light so asto close only front contact 54 of the level monitor 53, the relay C1becomes picked up by energization through front contact 54 and backcontact 55 of level monitor 53, contact 62 of switch 57, diode 63 andwire 58. The indicator lamp 0 is extinguished because of the opening ofback contact 54, and the lamp 1 is energized in an obvious mannerbecause of its connection to the circuit just described. If trafficcongestion is medium, both contacts 54 and 55 of level monitor 53 areclosed and the relay C2 is energized over wire 59 through front contacts54 and 55 and back contact 56 of level monitor 53, contact 64 of switch57 and diode 65. The lamp 2 of indicator 60 is energized in accordancewith its connection to this same circuit. If heavy congestion on thefreeway is detected, front contacts 54, 55 and 56 of the level monitorare all closed to provide for energization of both relays C1 and C2.Relay C1 is energized through contact 62 of switch 57 and diode 67, andrelay C2 is energized through diode 68. Thus the registration of lighttraffic congestion causes the picking up of relay C1 with the relay C2in its dropped way position, the registration of medium congestionresults in the picking up of relay C2 but not relay C1, and theregistration of heavy congestion results in the picking up of bothrelays C1 and C2.

Signal control relays C1 and SC2 are provided for the control of signal50. Relay 8C1 controls the aspects of signal 50 only during thetransition period from a normally proceed m de to a normally stop mode.

A metering timer 69 is provided for timing the duration between proceedintervals of the signal 50' when traffic congestion on the freeway hasbeen registered. A start timer 70 is provided for timing a yellow aspectof the signal 50 which is displayed for a time interval when the systemis shifted from a normally proceed mode to a normally stop mode. Vehicledetectors 1 and 2 are provided in the access ramp at positions torespectively sense the presence of vehicles in approach of the signal 50and after having passed the signal 59. Detector relays D1 and D2 areassociated with the detectors 1 and 2, and these relays become actuatedin response to the sensing of the presence of vehicles by theirassociated detectors.

To consider the specific mode of operation of the system as illustratedin FIG. 2, it will first be assumed that the system is in a normallyproceed mode with no traflic congestion having been registered as isindicated by the condition of the apparatus as illustrated. Under theseconditions the control relay SC2 is maintained steadily energized by acircuit including back contact 71 of relay C1, back contact 72 of relayC2 and diode 73. With relay SC2 steadily energized, the green lamp G ofsignal 50 is maintained steadily energized to permit steady flow oftraffic from the access ramp to the freeway.

When traffic congestion is registered by the picking up of either one orboth of the relays C1 and C2, the circuit for relay SC2 becomes opened,and energy is applied to one of the inputs to the metering timer 67 andto the input of the start metering time-r 70. If congestion is light andthe relay C1 is picked up and relay C2 is in its dropped away position,for example, energy is applied to input wire 74 of the metering timer 69through front contact '71 of relay C1 and back contact 75 of relay C2.Energy from this circuit is also applied as an input to the startmetering timer 70 through diode 76. Energy from this circuit also isapplied to the signal control relay SCl through back contact 77 of startmetering timer 70. Relay SCI remains picked up until the end of aninterval which has been started by the energization of the input wire ofthe start metering timer 70 and which is terminated when the startinterval timing is completed upon the opening of back contact 77.

Upon the picking up of relay SCI, the opening of back contact 52 causesthe green lamp G of signal 50 to be extinguished, and the clOsure offront contact 52 causes the energization of the yellow lamp Y of signal50 to indicate to approaching traffic that the control of the signal isbeing shifted from the normally proceed mode to the normally stOp mode.The energization of the yellow lamp Y is maintained effective until thestart metering timer has completed the timing of its interval, at whichtime the opening of back contact 77 causes the dropping away of relaySCI to open front contact 52 in the circuit for the yellow lamp Y ofsignal 50.

Because of the picking up of the relay C1, the circuit by which therelay SC2 has been energized is opened at back contact 71 of relay C1,and thus the relay SC2 is dropped away to close its back contact andthus condition a circuit for the energization of the red lamp R ofsignal 50 when the relay SCI becomes dropped away. Such energization ofthe red lamp R is in accordance with putting the system into a normallystop mode, and the energization of this lamp is maintained effective fora duration timed by metering timer 69. At the end of this interval,which is selected by the particular input wire that has been energized,the front contact 78 of metering timer 69 becomes closed, and theclosure of this c ntact permits the relay SC2 to become picked up whenthe detector 1 senses the presence of a vehicle in the immediateapproach of the signal 50. At this time the relay D1 becomes picked up,and with the relay D2 in its dropped away position, the relay SC2becomes energized through back contact '79 of relay D2, front contact 80of relay D1 and front contact '78 of metering timer 69.

The shifting of contact 51 of relay SC2 from its back to its frontposition extinguishes the red lamp R and energizes the green lamp G ofsignal 50 to start a proceed interval wherein the vehicle in approach ofsignal 50 is permitted to proceed. When the detector 2 senses that thevehicle has passed signal 50, the relay D2 becomes momentarily pickedup, and the picking up of this relay charges a capacitor 81 throughfront contact 82, so that the metering timer 69 becomes reset by thedischarge of capacitor 81 through back contact 82 when the relay D2becomes dropped away. Relay SC2 is deenergized upon the picking up ofrelay D2 by the opening of its circuit at back contact 79, and theresetting of the metering timer 69 as has been described prevents there-energization of the relay SC2 through front contact 78 until themetering timer has completed another cycle of timing. The start meteringtimer 70 becomes reset only when the system shifts from a normally stopmode to a normally proceed mode in accordance with there being noregistration of congestion of traflic on the freeway. Under theseconditions, the resetting of the timer 70 is rendered effective throughback contacts 71 and 72 of relays C1 and C2 respectively.

Having thus described specifically the mode of operation of the systemaccording to FIG. 2 for timing one particular selected interval inaccordance with one degree of traffic congestion, it should beunderstood that a similar mode of operation is effective when otherdegrees of congestion are registered, the metering timer 69 having itstiming adjusted in accordance with such degrees of congesti n inaccordance with energy applied selectively to the plurality of inputcontrol wires for the metering timer 69.

With reference to FIG. 3, another embodiment of the present invention isillustrated wherein the general mode of operation is maintained that hasbeen described for the system according to FIG. 2, except that the rampcontrol signal 85 of FIG. 3 is a two indication signal as compared tothe three indication signal 50 of FIG. 2. In the system according toFIG. 3, rather than providing the special yellow indication when thesystem shifts from a normally proceed mode to a normally stop mode, a 1teams is provided for shifting at a time when it is sensed that notrafiic is in the immediate approach of the signal 85, and in thismanner there is no shift from steady green to red of the signal 85 whena vehicle is in immediate approach of the signal.

In FIG. 3 the start metering timer 70 is controlled through a backcontact 86 of a detector relay D3, which registers the sensing ofvehicle traific by a detector 3 disposed in the access ramp at a pointupstream from the signal 85. The purpose of this detector is to delaythe change in mode in the control of the signal 85 until there is novehicle in the immediate approach of the signal 85. This mode ofoperation is accomplished by permitting the start metering timer 70 tooperate only provided that the back contact 86 of detector relay D3 isclosed. Thus the start metering timer 70 of FIG. 3 becomes stoppedwhenever a vehicle is detected by the detector D3 as being in approachof the signal 85. The start metering timer 70 is maintained in itsoperated position once it has completed its timing operation, as long asenergy is applied from a relay C1 or C2 in acordance with congestion onthe freeway. This is provided by the connection of front contact 77 ofthe timer 70 in multiple with back contact 86 in the input circuit forstart metering timer 70. Until such time as the start metering timer 70completes its timing operation, the relay SC3 is maintained energized inaccordance with energy feeding from relays C1 and C2 through backcontact of the start metering timer 7%. Thus the relay SC3 maintainsenergy on the green lamp G of signal 85 through front contact 51 untilan interval is completed by the start metering timer 70.

The mode of operation during the normally stop mode for the apparatusaccording to FIG. 3 is the same as has been described relative to FIG.2, the metering timer 69 being reset upon sensing the passage of avehicle past the signal 85, and the start metering timer 70 being resetwhen it is registered that there is no trafiic congestion on the freewayin accordance with the relays C1 and C2 both being operated to theirdropped away positions.

Having thus described several embodiments of the present invention asapplied to the control of trafiic en.- tering a freeway from an accessramp, it is to be understood that these forms have been selected for thepurpose of facilitating disclosure of the present. invention rather thanto limit the number of forms the present invention may assume. While theinvention has been described in its preferred embodiments, it is to beunderstood that the words which have been used are words of descriptionrather than of limitation and that changes wi.hin the purview of theappending claims may be made without departing from the true scope andspirit of the inven;ion in its broader aspects.

What I claim is:

1. In a system for controlling the flow of traffic onto a controlledaccess highway from an entrance ramp, :1 trafiic signal for selectivelydisplaying proceed and stop indications for controlling the passage ofvehicles from the ramp onto the highway, first vehicle detectin" meansfor providing an output characteristic of traffic congestion on thehighway, second vehicle detector means for providing an output inresponse to a vehicle on the ramp having passed the trafiic signal, andcontrol means governed by the output of the first detector means forselecting different times of initiation of a proceed indication of thesignal in accordance with respectively different degrees of congestionof tratiic on the highway, the control means being operatively governedby the output of the second detector for initiating a stop indication ofthe trafiic signal in response to the detection of a vehicle havingpassed the trafiic signal.

2. The invention according to claim 1 wherein the signal is governed bythe control means to substantially continuously provide :1 proceedindication except when traffic congestion is detected on the highway bythe first detector means.

3. The invention according to claim 1 wherein the signal is governed bythe control means so as to be put to stop each time a vehicle isdetected by the second detector, provided that tratfic congestion on thehighway is de tected by the first detector.

4. A system for regulating entry of vehicles from an access ramp onto aroadway in accordance with trafiic congestion on the roadway comprising,first vehicle detecting means sensing traffic on the roadway forproviding an output characteristic of trafiic congestion on the roadway,a traffic signal for designating intervals during which vehicles maymove from the ramp onto the roadway, second vehicle detecting means forgenerating an ou-put in response to sensing the presence of a vehiclewhich passes the signal to enter the roadway, first control meansgoverned by the output of the first vehicle detecting means forcontrolling the duration between intervals of the traffic signal todifferent values selected in accordance with different degrees ofcongestion of traffic on the roadway, and means controlled by the secondsensing means for at times limiting the duration of the intervals.

5. A system for controlling a tratfic signal governing ramp trafficentering a roadway selectively in normally stop and normally proceedmodes comprising, first vehicle detecting means sensing trafiic on theroadway for reg istering whether or not there is trafiic congestion onthe roadway, second vehicle detecting means for registering the presenceof traffic in approach of the trafiic signal, control means foroperating the trafi'ic signal in a normally stop .mode to initiate aproceed indication only after a vehicle has registered its presence byactuation of the second detecting means when trafiic congestion isregistered by the first detecting means, said control means beingeffective to operate the traffic signal in a normally proceed mode forpermitting continuous flow of traffic from the ramp to the roadway if notraffic congestion is registered by the first vehicle detecting means,and third vehicle detecting means including a vehicle detector along theramp between the signal and the roadway for resetting the signal to stopin response to passage of a vehicle past the signal when the controlmeans for the traffic signal is in a normally stop mode.

6. The system according to claim 5 wherein timing means is renderedeffective upon changing from a normally proceed to a normally stop modefor governing the control of the signal to time an internal between atime at which the normally proceed mode is terminated and a time atwhich the normally stop mode can be initiated.

7. The system according to claim 6 wherein the signal provides adistinctive indication during the interval timed by the timing means.

8. The system according to claim 7 wherein an aspect displayed by thesignal for the distinctive indication is of a color different thanaspects used for proceed and stop indications.

9. A system for regulating entry of vehicles from an access ramp onto aroadway in accordance Wi.h trafiic congestion on the roadway comprising,first Vehicle detecting means providing an output proportional totraffic congestion on the roadway, a trafiic signal designatingintervals during which vehicles may move from the ramp onto the roadway,second vehicle detecting means situated to sense vehicles passing thetrafiic signal, means coupling the first vehicle detecting means to thetrafiic signal for defining minimum length of the intervals of thetraffic signal in accordance with roadway trafiic congestion, andswitching means coupling the second vehicle detecting means to thetrafiic signal initiating prohibitory indications in response tovehicles passing the traffic signal.

10. The system according to claim 9-wherein the switching means isrendered operable to control the trafiic signal only after completion ofthe minimum length of each of the intervals.

References Cited UNITED STATES PATENTS 3,304,539 2/1967 Auer 34036THOMAS B HABECKER, Primary Examiner.

